A nickel-base alloy which has been used quite successfully in aircraft engine applications is disclosed and claimed in U.S. Pat. No. 3,615,376, issued Oct. 26, 1971 to Earl W Ross and assigned to the assignee hereof. A nickel-aluminide-type coating provides adequate protection from oxidation at high temperatures for buckets and other jet engine parts made of this alloy, but it has not been found that under substantially more severe environmental conditions resulting from use of alkali-metal-containing distillate or treated residual fuels, castings of articles of this alloy suffer catastrophic, localized corrosion or pitting. This mode of attack is unique among commercial nickel-base superalloys and is totally different from the hot corrosion attack which is characteristic of aircraft jet engine operation.
The merit of this special alloy compared to other nickel-base alloys is such that if this tendency toward pitting could be effectively eliminated even at the cost of some loss of overall hot corrosion resistance, the resulting alloy would still be very attractive for certain gas turbine applications.
This invention in both its method and article aspects is predicated upon several discoveries we have made and a basically new concept rooted in those discoveries. Thus, we have found that the localized, catastrophic, hot-corrosion attack to which nickel-base alloys of this type are vulnerable is associated with the localized concentrations of molybdenum and tungsten in the MC carbide phase. When these carbides are present at the surface of the alloy casting article, pitting can be initiated at those MC carbide sites. We have further found, however, that the amounts of molybdenum and tungsten are highly critical in producing this unusual corrosion effect and that the pitting tendency is effectively eliminated by reducing the combined total of molybdenum and tungsten in the carbide phase to less than about 15 percent. Additionally, we have found that such limitation of those two elements in the carbide phase can be readily accomplished without offsetting disadvantage or penalty to any desirable characteristic of the alloy through the addition of tantalum, columbium or hafnium or mixtures thereof. If used in combination, the amounts of these elements should aggregate in the range of 1.5 to 3.5 percent. If used individually, however, the amounts of these elements should preferably be in the following ranges:
These maxima and minima are governed by microstructural stability and carbide control considerations, respectively.
These findings have led to the new concept of displacing molybdenum and tungsten from the carbide phase to the alloy matrix to bring the total of these elements as carbides below the critical upper limit. In method or process terms, this concept is implemented through the new step of adding the requisite amount of the displacing element to the alloy suitably, but not necessarily, at the melt stage.
Still further, we have found that by maintaining the aluminum to titanium ratio in these new alloys relatively low and by limiting their carbon and zirconium contents, it is possible to have in these alloys an unique combination of desirable properties including superior resistance to ordinary hot corrosion and castability enabling their use in the production of large land-based utility gas turbine bucket castings. Particularly, the ratio (Al/Ti) should be approximately three to five (3/5), the carbon content should be from 0.08 to 0.13 at most and the zirconium content should be 0.02 to 0.07 at most.
Another important discovery is that zirconium is not an essential component or constituent of these alloys. Thus their superior resistance to localized pitting and to ordinary hot corrosion is not diminished by eliminating zirconium from them. Likewise, the unique castability characteristic enabling their use in the production of cast, large heavy duty gas turbine components such as gas turbine buckets is not detrimentally effected by not adding zirconium to, or including zirconium in, the alloys. The maximum zirconium content of these alloys, however, remains unchanged at about 0.07% as originally set forth in patent application Ser. No. 489,408 and detailed herein below.
A cast, nickel-base, superalloy article of this invention has an unique combination of mechanical properties, microstructural stability characteristics and resistance to localized pitting. In a preferred form, this article also has resistance to general hot corrosion and is readily produced as a sound casting in the form of a large land-based utility gas turbine bucket. These characteristics are attributable to the unique composition of the alloy and also to the method by which it is produced. In compositional terms, the alloys of this invention combining all the above desirable properties consist essentially of 13.7 to 14.3 percent chromium, 9 to 10 percent cobalt, 4.8 to 5.2 percent titanium, 2.8 to 3.2 percent aluminum, 2.8 to 4.3 percent tungsten, 1.0 to 1.5 percent molybdenum, 0.005 to 0.2 percent boron, 0.02 to 0.07 percent zirconium, 0.08 to 0.13 percent carbon, and 1.5 to 3.5 percent of tantalum, columbium or hafnium or mixtures thereof, or 2.0 to 3.0 percent tantalum, or 1.0 to 1.5 percent columbium, or 2.0 to 2.5 percent hafnium, balance nickel. Further, these alloys in the form of castings or articles such as gas turbine buckets are characterized by a matrix, a precipitate, and a monocarbide (MC) phase which consists of tantalum, titanium, molybdenum and tungsten in proportions such that the combined total of molybdenum and tungsten in the carbide constitutes less than about 15 percent of the carbide phase.
Alternatively, as indicated above, alloys of this invention having all those desirable properties in combination differ in composition from that stated just above in that they contain essentially no zirconium, that is, they are substantially free of zirconium, being formuated without addition of zirconium and therefore contain no more than trace amounts thereof.
According to the method of this invention, the novel nickel-base superalloy article generally described above is made by first preparing an ingot of the composition in the amounts stated above. As a second step, the ingot is remelted and cast to a form of the size and shape of the desired article. As the final step in the production of the preferred form of the article of this invention, the article is heat-treated in a vacuum or a neutral atmosphere, suitably in accordance with the appropriate time and temperature heat-treatment schedule.